Discovery of chromium catalysts led to the synthesis of linear high density polyethylene (HDPE) in the 1950's that to this day is still being manufactured. Although such HDPE's have broad molecular weight distributions (Mw/Mn typically greater than 6) they still don't have sufficient processability, shear thinning or high melt strength; and such HDPE's tend to have undesirably high die swell for certain applications such as for films and for pipes. Post-reactor oxygen tailoring has been employed to introduce a small amount of long chain branching in such HDPE's for film and blow molding applications, but the amount of branching is typically insufficient for pipe extrusion needs (see U.S. Pat. No. 5,728,335 and U.S. Pat. No. 5,739,266, and 15 MACROMOLECULES, 1460 (1982)).
One method to further enhance processability while raising the toughness in HDPE is to produce bimodal HDPE where the high molecular weight component contains a small amount of co-monomer (typically hexene or octane) while the low molecular weight component contains little or no co-monomer. Although these bimodal HDPEs can be used for pipe applications, their processability could still be further improved in order to raise the extrusion rate, reduce sag and die swell, and prevent ovality during extrusion.
Introduction of single site metallocene catalysts for the synthesis of polyethylene does not alter the coordinative insertion polymerization mechanism of earlier catalyst systems, such as chromium oxide or titanium chloride, and does not inherently create long chain branches unless the catalyst system is specially tailored. As first reported in EP 0 662 980 A1, and later in 29 MACROMOLECULES, 960 (1996), and 128 J. MOL. CATAL. A: CHEM., 65 (1998), long chain branches can be created in metallocene catalyzed ethylene polymerization by a copolymerization route in which in-situ generated vinyl/vinylidene-terminated polyethylene is incorporated into growing polymer chains. For this to occur, the metallocene catalysts need to have high vinyl selectivity (prefer beta hydride elimination for vinyl chain ends) and good copolymerization capability. Also, long chain branched polyethylene can be produced with mono-cyclopentadienyl metallocene, as well as certain C2 symmetric metallocenes. Both supported and homogenous catalysts can be employed for long chain branched polyethylene synthesis and they can be made in gas phase, solution, or slurry reactors.
There are, however, two major issues with prior approaches in synthesizing long chain branched polyethylene by metallocene. One is that the reinsertion frequency of vinyl/vinylidene-terminated polyethylene is very low during polymerization which leads to lightly branched chains. With low branching density, it is then required to have many long chain branched molecules in the final products, typically greater than 30 wt % (balanced by linear molecules), in order to see any processability improvements. Further, addition of long chain branched polyolefins into a linear polyolefin can lead to an erosion of the linear polyolefin's toughness especially, when larger amounts are used. This is believed to be due to the reduction of polymer coil dimension in the presence of the long chain branches.
In accordance to Huang-Brown tie chain theory (Y. L. Huang and N. Brown, 29 J. POLYM. SCI. POLYM. PHYS. ED., 129 (1991)), maintaining the crystallize size while reducing the coil dimension leads to fewer tie chains in-between crystallites, as well as lower toughness. For instance, the dart impact toughness of long-chain branched Enable™ (ExxonMobil Chemical Company) is only about 25 to 35% of the dart impact value of linear Exceed™ LLDPE (ExxonMobil Chemical Company) with similar molecular weights. Thus, there is still a need to introduce a highly long chain branched HDPE.
References of interest include US 2004/214953; US 2005/065286; US 2009/318640; and US 2014/039140.